The colonnade - that is the word for the curved rows of columns - was designed by a brilliant Italian sculptor and architect named Gian Lorenzo Bernini, who started building it in 1656. Bernini said he wanted the columns to look like the arms of a mother hugging her children. It took eleven years to finish.
Each column is about 13 metres tall - roughly the height of four double-decker buses stacked on top of each other. There are 284 columns arranged in four rows, and 140 statues of saints stand along the very top, looking out over the square. If you tried to count every single stone piece in the colonnade, you would be counting for a very long time.
Right in the middle of the square stands a tall, pointed stone called an obelisk. It was brought to Rome from Egypt nearly 2,000 years ago, and it is even older than that - carved about 4,000 years ago. Two fountains gurgle on either side of it, and pigeons love to splash in them on sunny afternoons.